Various types of couplings are disclosed for connecting tubes together but each of them suffer from at least one major drawback.
A typical way of coupling tubes together, is to use a bracket which is preformed in two halves so that when the halves are combined they define two holes which are arranged to receive different tubes. Using nuts and bolts, the brackets are tightened together so as to grip each of the tubes and thus, hold the tubes in a fixed orientation with respect to each other.
The problem with such a bracket is that it is necessary to hold the tubes in a fixed orientation before the two halves of the bracket are combined and in addition, there is little flexibility as to changing the orientation of each tube with respect to the other. This is because the brackets must be preformed so that when connected to tubes they can only hold the tubes in one particular orientation with respect to each other.
Other devices for connecting tubes together require the tubes to be specially shaped so that they have flattened ends in the form of lugs which can be inserted onto the end of a coupling device. A combination of brackets and nuts and bolts are then used to assemble the tubes into a desired structure.
Another form of a coupling device is in the form of a joint with preformed sockets arranged around a centre, thus, permitting tubes to be connected in one of the orientations provided by one of the sockets. As with the previous coupling devices however, there is little flexibility in the number of positions in which the tubes can be fixed.